Clarify role of <com:Link> in SDMX-ML 3.0.0

[khaeru]

In the wild, SDMX-ML 3.0.0 from IMF (see specimen here) contains:

…
<str:Codelist urn="urn:sdmx:org.sdmx.infomodel.codelist.Codelist=IMF:CL_FREQ(1.0.3)" agencyID="IMF" id="CL_FREQ" version="1.0.3" isExternalReference="false" isPartial="false">
  <com:Link url="urn:sdmx:org.sdmx.infomodel.codelist.Codelist=IMF:CL_FREQ(1.0.3)" type="codelist" rel="self"/>
  …

In SDMX 3.0.0:

• The IM (§3.2.2.2, page 31) specifies the following:
  • IdentifiableArtefact.uri: Universal resource identifier that may or may not be resolvable.
  • IdentifiableArtefact.urn: Universal resource name – this is for use in registries: all registered objects have a urn.
• The class diagram (§3.2.1, page 29) shows these as having type "url" and "urn" respectively, with cardinality of 1.
• The SDMX-ML 3.0.0 schemas (here) show:

  <xs:element name="Link" type="LinkType">
    <xs:annotation>
      <xs:documentation>Allows for the linking of other resources to identifiable objects. For example, if there is reference metadata associated with a structure, a link to the meatadata report can be dynamically inserted in the structure metadata.</xs:documentation>
    </xs:annotation>
  </xs:element>

  …and further that this element has the mandatory url attribute and optional urn attribute.
• The schemas also show, in many places:

  <xs:element ref="common:Link" minOccurs="0" maxOccurs="unbounded"/>
  

  In other words, there can be arbitrarily many <com:Link> elements within a parent element representing an IdentifiableArtefact

Generally, "URN" and "URL" are both subcategories of "URI".

There are multiple things to clarify here:

1. Is IdentifiableArtefact.uri supposed to be, per its name, any URI (so possibly a URL or URN) or only (per its type) a URL?
2. Is a <com:Link url="…" rel="self"> supposed to correspond to IdentifiableArtefact.uri?
3. Is SDMX-ML that contains multiple <com:Link> (as allowed by the XSD) valid? Which should correspond to IdentifiableArtefact.uri?
4. Is the IMF usage valid in which a URN, not a URL, appears in the url="…" attribute?

Pending clarification, #227 ignores <com:Link> with a log message.